The invention relates to a bone anchoring device comprising a shank to be anchored in a bone or a vertebra and a receiving part connected to the shank for receiving a rod. In particular the invention relates to a polyaxial bone screw wherein the head of the screw and a rod can be fixed independently and which has a reduced size.
U.S. Pat. No. 6,224,598 B1 discloses a threaded plug closure adapted for use in securing a rod member to a bone screw implant, said closure comprising a plug having a threaded cylindrically-shaped outer surface, said plug being received between a pair of arms of a medical implant during use, a central coaxial bore passing entirely through that plug, said central bore having an internal threaded surface which is shaped to receive a set screw. The plug closure and the set screw can be independently installed and the set screw tightened to cooperatively provide capture and locking of the rod in order to secure the rod against translational and rotational movement relative to the bone screw.
US 2003/0100896 A1 discloses a bone anchoring device with a shank and a receiving part connected to it for connecting to a rod. The receiving part has a recess having a U-shaped cross section for receiving the rod forming two open legs. An internal thread is provided on the open legs. A locking assembly is provided comprising a nut member with an external thread which cooperates with the internal thread of the legs and a set screw. The nut member has on one end slits for engagement with a screw tool. The shank has a spherically-shaped head which is pivotably held in the receiving part and a pressure element is provided which exerts pressure on the head when the nut member is tightened. By tightening the set screw the rod is fixed in the receiving part. Hence, the rod and the head can be fixed independently from each other. The internal thread and the cooperating external thread of the nut member are designed as a flat thread. The implant has a compact design, since an outer ring or nut to prevent splaying of the legs is not necessary.
U.S. Pat. No. 6,063,090 discloses a bone anchoring device with a locking assembly consisting of two parts, a tensioning screw fixing the spherical head via an insert without jamming the rod and a tension means fixing the rod.
US 2006/0036244 A1 also discloses a bone anchoring device with a two part locking cap for fixing the head and the rod independently. The tensioning screw has a coaxial recess with a structure for engagement with a tool.
The outer diameter of the locking device is under various aspects determined by the required tightening torque and the thread form. In turn, the overall dimensions of the upper portion of the bone anchoring device are determined by the size of the locking device. For certain applications, it is crucial to have the distance between one bone anchoring device and a neighboring bone anchoring device as small as possible, for example for correcting scoliosis in infants or for the application in the cervical region of the spine.
Based on the foregoing, there is a need for a bone anchoring device with a locking device which has the same reliability as the known devices but which has significantly smaller dimensions of the upper portion in the direction of the rod axis.